The finding of two fetal cell vitamin A-compatible binding sites, one for retinol and another for retinoic acid, but only one adult binding site (which is for retinol) appears to indicate that there are two functional (apparently non-interchangeable) forms of vitamin A. These findings indicate that in certain fetal tissues (and in perhaps tumor tissue) retinol is converted to retinoic acid to satisfy any requirement for this compound, since the compound does not occur in nature. Hence, one of the problems, which will be studied in depth with several different tissues, is the possible conversion of retinol to retinoic acid. The other area, in which much further study will be carried out, is the area of cell culture and the finding of cell types which require retinol and/or retinoic acid for growth, development, or differentiation. A number of cell types are being investigated. These types include various cells from normal animals as well as certain transformed cells.